1. Field of the Invention
The present invention relates to a connection method and a connecting structure for connecting a photonic crystal fiber and an optical fiber of a different type or two photonic crystal fibers with low loss.
Priority is claimed on Japanese Patent Application No. 2004-183639, filed Jun. 22, 2004, the content of which is incorporated herein by reference.
2. Description of Related Art
A photonic crystal fiber is an optical fiber that has microholes in the cladding region thereof and is capable of exhibiting characteristics that cannot be achieved with a conventional optical fiber having a core/cladding structure. Such photonic crystal fibers have been developed as promising functional fiber of various types or transmission fiber for the future. Since a cladding region of a photonic crystal fiber has a number of microholes, the cladding region equivalently has a lower refractive index than that of the core region and totally reflects light coming from pure silica of the core region, thereby making the light propagate through the core region. By changing the design of the arrangement of microholes, it is possible to obtain various characteristics.
In order to use such photonic crystal fibers as transmission fibers or fibers employed in various optical components, it is required to connect a photonic crystal fiber and an optical fiber of a different type or two photonic crystal fibers with low loss.
Conventionally, as a method for connecting a photonic crystal fiber and an optical fiber of a different type or two photonic crystal fibers with low loss, a technique disclosed in Japanese Unexamined Patent Application, First Publication No. 2002-243972 has been proposed.
In addition, as a method for improving the connection strength between photonic crystal fibers, a technique disclosed in Japanese Unexamined Patent Application, First Publication No. 2004-77890 has been proposed.
A photonic crystal fiber has a core region formed from silica glass that has the same refractive index as that of a cladding region, and the cladding region has a number of microholes. When such a photonic crystal fiber and an optical fiber of a different type or two photonic crystal fibers are fusion-spliced, if a number of microholes in the cladding region collapse upon splicing, the collapsed portion becomes silica glass portion having a uniform refractive index profile in which the wave-guiding structure is destroyed, resulting in an increase in the splice loss. In order to reduce the splice loss, it is required to connect fibers while preserving the microhole structure so that the wave-guiding structure is not destroyed. For preserving the microhole structure so that the wave-guiding structure is not destroyed, the heating time and the heating temperature must be minimized. However, reducing the time and the temperature for heating a fusion-spliced portion leads to the insufficient fusion, resulting in reduced splice strength.
In the connection method disclosed in Japanese Unexamined Patent Application, First Publication No. 2002-243972, a fusion is performed by sealing microholes of a photonic crystal fiber. When the microholes are sealing of, the sealed portion has the same structure as that of a fiber to be connected, which helps to achieve significant reduction in the splice loss. In addition, since the mode field diameter of the connecting end portion of the photonic crystal fiber is enlarged, it is possible to reduce the splice loss by connecting the photonic crystal fiber to a fiber having a large mode field diameter. However, when this connection method is used for connection of a photonic crystal fiber that includes a core region having the same refractive index as that of a cladding region, in the portion in which microholes collapse, the core and the cladding are fused as a single body in which the wave-guiding structure is destroyed. For this reason, this connection method cannot be used for achieving a fusion splice of a photonic crystal fiber that includes a core region having the same refractive index as that of a cladding region with low loss.
In the connection method disclosed in Japanese Unexamined Patent Application, First Publication No. 2004-77890, in order to prevent microholes of photonic crystal fiber from being collapsed upon fusion splicing, the spacing between fiber end faces are reduced to minimize the effect of the heat to fiber end faces, thereby preventing the microholes from collapsing. It is possible to reduce the heat applied to the fiber end face using this method, and Japanese Unexamined Patent Application, First Publication No. 2004-77890 teaches that a strong fiber connecting structure can be obtained by pressing the fiber to a predetermined length. However, the improvement in the splice strength achievable by this connection method is insufficient.